Llandovery first appearances

Arachnids are arthropods in the class Arachnida () of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroons.

Placoderms (from Ancient Greek πλάξ [plax, plakos] 'plate' and δέρμα [derma] 'skin') are vertebrate animals of the class Placodermi, an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Devonian periods. While their endoskeletons are mainly cartilaginous, their head and thorax were covered by articulated armoured plates (hence the name), and the rest of the body was scaled or naked depending on the species.

thumb|254px|right|Anaspids are characterized by a large, tri-radiate spine (red) posteriorly to the series of branchial openings. It is assumed that the most primitive anaspids, such as Pharyngolepis (top), possessed a long, ribbon-shaped, [[ventrolateral fin-fold (green). More advanced forms, such as Rhyncholepis (bottom), possessed a shorter paired fin-fold (green) and enlarged, spine-shaped, median dorsal scutes. – Philippe Janvier]] Anaspida ("shieldless ones") is an extinct group of jawless fish that existed from the early Silurian period to the late Devonian period. They were classi

first series and epoch of the Silurian system and period

Adelophthalmoidea (the name deriving from the type genus Adelophthalmus, meaning "no obvious eyes") is a superfamily of eurypterids, an extinct group of aquatic arthropods. Adelophthalmoidea is classified within the infraorder Diploperculata, in the suborder Eurypterina. The superfamily contains four families: Adelophthalmidae, Nanahughmilleriidae, Parahughmilleriidae, and Pittsfordipteridae.''''''

Pterygotidae (the name deriving from the type genus Pterygotus, meaning "winged one") is a family of eurypterids, an extinct group of aquatic arthropods. They were members of the superfamily Pterygotioidea. Pterygotids were the largest known arthropods to have ever lived with some members of the family, such as Jaekelopterus and Acutiramus, exceeding 2 metres (6.6 ft) in length. Their fossilized remains have been recovered in deposits ranging in age from 428 to 372 million years old (Late Silurian to Late Devonian).

Rhynchonella is an extinct genus of brachiopods known from the Late Jurassic (Oxfordian) to the Early Cretaceous (Valanginian, possibly Barremian). Formerly this genus was understood much more widely (more or less an equivalent of the Rhynchonellida order in the present-day taxonomy) and less critical sources still list species of Rhynchonella from the Ordovician to the Eocene. Like most brachiopods, Rhynchonella was a stationary epifaunal suspension feeder.